Epoxy resins are well known in the art to have improved electrical properties, humidity resistance and heat resistance. Among others, epoxy resin compositions of the acid anhydride curing type are widely used in encapsulating optical semiconductor devices because of their light transparency.
Often anti-discoloring agents are added to such light transmissive epoxy resin compositions in order to prevent them from changing their color toward brown upon high-temperature treatment into cured products. The known anti-discoloring agents are organic phosphorus compounds, hindered phenols and thioethers. The inventors have found that the organic phosphorus anti-discoloring agents are most effective among these agents.
It is also a common practice to produce low stressed epoxy resins compositions by blending inorganic filler such as silica therein, thereby reducing a coefficient of linear expansion.
Epoxy resin compositions having inorganic fillers such as silica loaded therein, however, cure to opaque products although both the epoxy resin and filler components are transparent. This is because most cured epoxy resins have a refractive index (n.sup.25.sub.D) of about 1.5 to 1.7 and its difference from the refractive index of filler (for example, N.sup.25.sub.D .apprxeq.1.458 for SiO.sub.2 causes light scattering. This suggests that if a filler having a refractive index approximate to that of cured epoxy resins were blended, cured products would be transparent.
In this regard, the inventors found that silica-titania glass having a high refractive index is an effective inorganic filler. In general, sol-gel methods are known for preparing transparent glass species having varying indexes of refraction. The sol-gel methods produce glass by starting with a solution of organic and inorganic compounds of metals, causing hydrolysis and polymerization of the compounds in the solution to form a sol having fine particles of metal oxide or hydroxide suspended, causing the reaction to proceed further to convert the sol into a gel, and heating the porous gel into an amorphous glass or polycrystalline body. The inventors previously proposed a method for preparing high transparency silica-titania glass beads in the above-referred application Ser. No. 07/651,438. This method produces silica-titania glass beads by furnishing a mixed solution of metal alkoxides (silicon alkoxide and titanium alkoxide), alcohol and water, gradually evaporating alcohol from the solution, causing hydrolysis and polycondensation to form a sol and then a wet gel, and drying the gel, followed by grinding and firing of the dry gel. By changing the mix proportion of silicon alkoxide and titanium alkoxide, silica-titania glass beads can be controlled to a desired refractive index, that is, match with the refractive index of cured epoxy resins.
It was expected that by blending such silica-titania glass beads in epoxy resin compositions as a filler, there would be obtained epoxy resin cured products high transparency, a low coefficient of linear expansion, and low stress.
In attempts to blend silica-titania glass beads in epoxy resin compositions along with organic phosphorus anti-discoloring agents which were found to be most effective in preventing the epoxy resin compositions from discoloring upon high-temperature treatment, it was found that the interaction between the organic phosphorus anti-discoloring agent and silica-titania glass beads gave rise to yellowing phenomena. Unlike the above-mentioned browning of cured products upon high-temperature treatment, serious color development occurred even at room temperature as long as both the components co-existed, resulting in a substantial lowering of light transmittance of the cured products.
The filled epoxy resin compositions also suffered from the problem that the cured products became turbid and low in light transmittance due to light scattering caused by separation and gaps occurring at the interface between the resin component and the filler.
Therefore, a mere combination of an organic phosphorus anti-discoloring agent with silica-titania glass beads failed to provide a light transmissive epoxy resin capable of meeting all the requirements of anti-discoloring upon high-temperature treatment, high transparency, and low coefficient of linear expansion. The filled epoxy resin compositions as such were impractical for semiconductor encapsulating purposes.